Differential gears are commonly used in watch movements with automatic or manual rewinding.
By way of example, patent CH 263 707, published on Dec. 1, 1949, describes a differential gear for the displaying of the running reserve of a watch.
This differential gear includes two large wheels provided, on the one hand, with radial toothings connected kinetically to one end and the other of the barrel spring and, on the other hand, with contrate toothings configured to engage a satellite having the form of a conventional pinion. The differential gears of this type are cumbersome in terms of their thickness and are therefore not used very often in wristwatches.
Moreover, a general problem with mechanisms with running reserve indication is management of a value to be displayed when the spring of the barrel is fully rewound, either through an automatic winding mechanism, or through manual winding. If the winding is continued, the barrel generally includes a slip-spring in order to prevent any potential damage, while a display mechanism of the running reserve includes a stop at a location or otherwise in order to prevent a display member from turning further. However, a wheel of the differential gear is connected kinetically to a barrel drum which continues to turn during the winding, even when the spring has been fully wound. It is therefore desirable to provide a system for preventing the gears involved in the display of the running reserve from any damage, in these conditions.
The above-mentioned patent proposes to mount one of the wheels of the differential gear on its shaft with a viscous friction in order to allow it to slide on the shaft when the barrel spring has been fully wound.
Note that such a measure further makes it possible to be free of the requirement of providing a pairing for the placing of the member indicating the running reserve due to the fact that its position can be adjusted by means of the stop mentioned above.
Patent application EP 1 139 182 A1, published on Oct. 4, 2001, proposes an alternating differential gear, implementing gears of a conical type, less cumbersome than those described above. Furthermore, that patent application proposes a solution, making it possible to take into account the situation put forth hereinabove, consisting in inserting a specific, but complex, mobile, between the differential gear and the mobile for displaying the running reserve.